Tidy up coursebook export

.../frontend/components/coursebook/CoursebookPrintDialog.vue

aleksis/apps/alsijil/model_extensions.py

aleksis/apps/alsijil/models.py

aleksis/apps/alsijil/tasks.py

...sis/apps/alsijil/templates/alsijil/partials/absences.html

aleksis/apps/alsijil/templates/alsijil/partials/legend.html

...pps/alsijil/templates/alsijil/partials/lesson_status.html

...lsijil/templates/alsijil/partials/lesson_status_icon.html

...pps/alsijil/templates/alsijil/partials/objects_table.html

...s/alsijil/templates/alsijil/partials/person_overview.html

...lsijil/templates/alsijil/partials/persons_with_stats.html

...sijil/templates/alsijil/partials/register_coursebook.html

.../apps/alsijil/templates/alsijil/partials/tardinesses.html

aleksis/apps/alsijil/urls.py

aleksis/apps/alsijil/views.py